Silicon carbide (hereinafter referred to as SiC) is superior in heat resistance and mechanical strength, besides it has good resistance to radiation. Further, it is easy to perform valence control of electrons and holes by doping with impurities. Furthermore, SiC has a wide band gap, for example, single crystal 6H—SiC has a band gap of about 3.0 eV and single crystal 4H—SiC has a band gap of 3.3 eV. Therefore, it is possible to realize high temperature, high frequency, withstand voltage, and environmental resistance properties, which can not be realized by any existing semiconductor material such as silicon (hereinafter referred to as Si) and gallium arsenide (hereinafter referred to as GaAs). SiC attracts attention and is expected as a semiconductor material for next-generation power devices and high-frequency devices. On the other hand, hexagonal SiC has a lattice constant close to that of gallium nitride (hereinafter referred to as GaN) and is expected as a substrate for GaN.
Conventionally, single crystal SiC of this type is produced by a sublimation and recrystallization method (modified Lely method) in which a seed crystal is fixedly placed on the lower-temperature side in a graphite crucible and SiC powder as a raw material is inserted in the higher-temperature side, and then the interior of the graphite crucible is heated to a high temperature of 1450 to 2400 degrees C. in an inert atmosphere, and thereby the SiC powder is sublimated and recrystallized on a surface of the seed crystal on the lower-temperature side to grow a single crystal. Otherwise, single crystal SiC is produced by an liquid phase epitaxial growth method (hereinafter referred to as LPE method) in which Si melt is put in a crucible containing carbon (hereinafter referred to as C) atoms, and then the Si melt is heated to the crystal growth temperature by heating the crucible and a single crystal SiC substrate supported by a holder or the like is dipped in a low-temperature region in the Si melt for a certain time, and thereby C as a constituent element of the crucible is dissolved in the Si melt and single crystal SiC produced by reaction between Si and C is epitaxially grown on a surface of the single crystal SiC substrate.
In the above-described conventional growth methods, however, in case of the sublimation and recrystallization method, although the growth rate is very high as several hundreds micrometer/hr, the SiC powder is once decomposed into Si, SiC2, and Si2C upon sublimation to evaporate, and further they react with part of the graphite crucible. Therefore, gas that reaches the surface of the seed crystal varies in kind in accordance with a change in temperature. It is technically very difficult to stoichiometrically accurately control the partial pressures of them. In addition, impurities are easy to mix in, and crystal defects, micropipe defects, etc., are apt to be generated under the influence of distortion caused by the mixed impurities or heat. Further, there is generation of grain boundaries caused by generation of many nuclei. Thus, there is a problem that single crystal SiC stable in performance and quality can not be obtained.
On the other hand, in case of the LPE method, there is less generation of micropipe defects, crystal defects, etc., as observed in the sublimation and recrystallization method, and single crystal SiC is obtained that is superior in quality in comparison with that produced by the sublimation and recrystallization method. However, as shown with black triangular marks in FIG. 6, the growth rate is very low as 10 micrometer/hr or less because the rate of the growth process is influenced by the solubility of C in the Si melt. Therefore, the productivity of single crystal SiC is low and the temperature of the liquid phase in the production apparatus must be accurately controlled. In addition, the production process is complicated and the production cost of single crystal SiC is very high. A method of mixing transition metal such as Sc in the Si melt may be adopted so that the solubility of C in the Si melt is increased to promote the growth rate. In this case, however, because the transition metal is taken in the growing crystal as impurities, the purity is deteriorated. Thus, single crystal SiC fully satisfactory in quality and performance can not be obtained. In the growth process, as shown with square marks in FIG. 6, the solubility of C in the Si melt is increased by mixing Sc in. However, there is a problem that the productivity of single crystal SiC is very low in comparison with the sublimation and recrystallization method.
The present invention has been made in view of the above-described problems and aims to provide high-quality, high-performance single crystal SiC in which generation of micropipe defects, interface defects, etc., is less and which has a broad terrace and high surface flatness.